This invention relates to control systems for electronically scanned, phased array antennas, and more particularly to a control system for an electronically scanned, phased array antenna which is rotatable about at least one axis of movement to enable positioning of the aperture of the antenna so as to maximize the total carrier-to-noise-plus-interference ratio of the signal received by the antenna from a primary signal source when at least one interfering signal source is present in the coverage region of the antenna.
Mobile platforms, especially aircraft, require compact, low profile antenna systems. Antennas that offer a combination of mechanical and electronic scanning are used in a number of applications. These antennas usually have a limited aperture size to meet size restrictions. Such size restrictions are often necessary, especially in aircraft, where weight and aerodynamics of the antenna are very important factors. Additionally, these antenna apertures frequently comprise a rectangular configuration due to production and cost considerations. This combination of limited aperture size and rectangular aperture configuration results in both a larger beamwidth and significant beam side lobe xe2x80x9ctrainsxe2x80x9d or xe2x80x9cridgesxe2x80x9d that are oriented along the primary axes of the antenna aperture.
The above-described side lobes are not a significant problem when the antenna is operated with a single source, such as when the antenna is receiving signals from a single satellite transponder within a given coverage region. However, when this type of antenna is operated in an environment where interfering signal sources (i.e., non-primary signal sources) are present, such as when one or more additional satellite transponders are present within the coverage region and in relatively close proximity with the primary signal source (i.e., on the order of several beamwidths), then the orientation of the antenna side lobes is critical. A typical control system for this type of antenna uses either navigation data to orient the antenna xe2x80x9cbroadsidexe2x80x9d to the satellite transponder being tracked (i.e., to the primary signal source) or uses a closed-loop, received power maximizing approach to achieve the same result. Either approach can have the effect of lining up the side lobe xe2x80x9ctrainsxe2x80x9d on the interfering signal sources.
The above-described scenario of an environment with significant interfering signal sources describes the situation for aircraft-type commercial Ku-band antennas operating with the most common (and lowest cost and most available) type of satellite services in this bandxe2x80x94those operated on Fixed Satellite Service (FSS) satellites. In this case, for antennas with conventional control systems, the lining up of the sidelobe trains with one or more interfering satellites along the geosynchronous arc can result in a significant loss of performance. This loss of performance is especially exacerbated when the antenna is operated at lower latitudes.
FIG. 1 shows performance estimates for a mechanically augmented phased array (MAPA) antenna with a conventional control system and geographically demonstrates the loss of performance at lower latitudes. Contour C7 represents 7 Mbps. Contours C6-C1 represent 6 Mbps to 1 Mbps in 1 Mbps steps.
FIG. 2 illustrates the case of a bore-sighted (i.e., zero scan angle) main beam. For a rectangular phased array antenna, a key feature is that as the antenna main beam is scanned, corresponding to moving the peak xe2x80x9cPxe2x80x9d to different points on the illustrated circular grid xe2x80x9cGxe2x80x9d, the sidelobe ridges (xe2x80x9cRxe2x80x9d) move with the main beam while remaining approximately parallel to the principal antenna axes. The location of the axes of the sidelobe ridges with respect to other satellite transponders or signal sources is an important determinant of interference levels. When these antennas are fixed on aircraft, the orientation of the sidelobes is a function of the aircraft""s direction of flight and cannot be controlled. When these phased array antennas are able to rotate, such as with a MAPA antenna, the additional degree of freedom can be used to favorably orient these axes with respect to interfering signals. This is in contrast to the traditional approach, which is to turn the antenna to face the satellite transponder (i.e., the primary signal source) as closely as possible in order to reduce scan loss and maximize received radio frequency (RF) power.
Accordingly, it is a principal object of the present invention to provide a control system and method for a MAPA antenna that controls pointing of the antenna aperture 50 that the antenna aperture is pointed not directly at a primary signal source, but rather is pointed at least slightly away from the primary signal source in a controlled manner to minimize the influence of interfering signal sources without significantly adversely affecting the strength of the received signal.
It is another object of the present invention to provide a MAPA antenna which is controlled by a control algorithm which monitors the signal quality of the signal received by the antenna and controls the positioning of the MAPA antenna in accordance with the determined signal quality, in a closed loop fashion, to minimize the influence of interfering signal sources within the coverage region of the antenna.
It is still another object of the present invention to provide a MAPA antenna wherein the position of the antenna is controlled by an open loop control system making use of a look-up table including antenna position coordinates for positioning the antenna at relatively precise positions based on prior known locations of interfering signal sources.
The above and other objects are provided by a control system and algorithm for use with a MAPA antenna to control pointing of the antenna aperture in a manner to minimize interference from non-primary signal sources operating within a coverage region of the antenna while the antenna is receiving signals from a primary signal source, and without significantly reducing the signal strength of the received signal. In one preferred form, a closed loop control algorithm is provided for determining the optimum positioning of the MAPA antenna to reduce the influence of interfering signal sources. This algorithm accomplishes rotation of the MAPA antenna off axis to point the antenna sub-optimally in terms of scan angle loss, but which provides significant gains in terms of reduced interference, thus resulting in overall improved antenna performance.
The closed loop algorithm approach makes use of a measure of signal quality of the received signal to control a motor used for positioning the MAPA antenna. Electronic steering of the MAPA antenna is accomplished using traditional power feedback or open-loop pointing methods. The signal quality may be represented by various factors, but in one preferred embodiment is represented by the bit error rate (xe2x80x9cBERxe2x80x9d) of the received signal. The BER is monitored and the MAPA antenna aperture""s face is rotated at least slightly away from the primary signal source while at the same time electronically steering the antenna beam back on target to the primary signal source with a conventional beam steering controller. Monitoring the BER makes it possible to maximize the total carrier-to-noise-plus-interference (xe2x80x9cCNIxe2x80x9d) ratio of the received signal, and thus significantly limit the influence of interfering signal sources.
In an alternative preferred form of the present invention, an open-loop control system is employed for controlling positioning of a MAPA antenna. This embodiment makes use of look-up tables incorporating pre-calculated antenna position information based on the positions of known, interfering signal sources within a coverage region. Longitude and latitude information of the moving platform (i.e., aircraft or ship) is used by the control system in this embodiment to determine optimal antenna positions (i.e., azimuthal positions) from the look-up tables so that pointing of the MAPA antenna is accomplished in a manner that minimizes interference from the interfering signal sources.
In another alternative preferred embodiment, an open loop control system is employed which makes use of real-time calculations using a simplified model of the positions of likely interfering signal sources. With this approach, it is assumed that the worst interfering, non-primary signal sources will lie approximately along the line of the normal to the geostationary arc at the longitude of the primary signal source being tracked. This normal line is calculated using suitable equations for calculating the horizontal axis angle of the primary signal source in antenna system coordinates. The MAPA antenna is then rotated to an orientation that puts this line off of the primary axes for the sidelobes of the MAPA antenna.
In each of the above-described embodiments, the antenna aperture is physically pointed not directly at the primary signal source, but rather at least slightly away from the primary signal source and then the beam is electronically steered to track the primary signal source. The pointing of the antenna at least slightly away from the primary signal source, but in a direction that minimizes the influence of signals received from interfering signal sources on the sidelobe trains of the antenna thus serves to maximize the CNI ratio of the received signal from the satellite being tracked.